1. Field of the Invention
The present invention relates to a voltage supply device which drives a relatively high voltage load such as an electroluminescent display panel, and more particularly to such a device that includes a circuit for testing its own operation under a test voltage that is considerably lower than a normal operation voltage.
2. Description of Related Art
An example of a voltage supply device for driving scanning and data electrodes of an electroluminescent display panel is disclosed in JP-A-8-137433. In this device, an output circuit for selectively outputting a high voltage and a ground voltage, essence of which is shown in FIG. 4, is used. An output circuit 3 composed of a P-channel MOSFET 1 and an N-channel MOSFET 2, both connected in series, is connected between a plus terminal +V and a ground terminal GND of a high voltage source. Both MOSFETs 1 and 2 constitute a push-pull circuit, and an output terminal 4 is connected to a junction of both MOSFETs. A gate voltage switching circuit 5 composed of plural resistances 6 and an N-channel MOSFET 7, all connected in series, is connected in parallel to the output circuit 3. A divider terminal 6a of the switching circuit 5 is connected to a gate of the MOSFET 1. An input terminal 8 is connected to a gate of the MOSFET 7, and another input terminal 9 is connected to a gate of the MOSFET 2.
The MOSFET 7 is turned on when a signal is fed from the input terminal 8, thereby bringing a voltage at the divider terminal 6a to a level that turns on the MOSFET 1 and is a little lower than the plus terminal voltage +V. When the MOSFET 1 is turned on, the voltage at the output terminal 4 becomes the plus terminal voltage +V. The MOSFET 2 is turned on when a signal is fed from the input terminal 9, thereby bringing the output voltage at the output terminal 4 to the ground level GND. The input signals fed from the input terminals 8 and 9 are relatively low, e.g., 5 V that is the same as an operating voltage of a control circuit. Thus, the output voltage of the output terminal 4 is selectively switched between +V and GND, and is supplied to electrodes of an electroluminescent display panel.
Usually, operation of the driver IC including the output circuit 3, i.e., whether the output from the output terminal 4 is selectively switched between +V and GND according to the input signals fed from the input terminals 8 and 9, is tested after completion of the driver IC. To test the operation of the conventional driver IC, it is necessary to supply a power source voltage that is as high as 150 V, for example. Therefore, the test is not easy, and a testing apparatus having a high voltage is required.